In TDMA-based cellular radio systems a base station allocates cyclically occurring time slots from certain transmission frames to the use of portable terminals. We will discuss the known GSM (Global System for Mobile telecommunications) and its further developed version EDGE (Enhanced Datarates for GSM Evolution) as examples.
As a background for the invention, the known GSM transmission chain will be briefly discussed with reference to FIG. 1. The transmission of full-rate speech is used here as an example of a typical service requiring a circuit-switched connection. Speech recorded by a microphone 101 will first be encoded in a speech encoder 102 which converts an analogue speech signal into digital form and performs a group of encoding operations. The output signal of the speech encoder has a rate of 13 kbit/s and consists of blocks of 260 bits, the blocks following each other at an interval of 20 ms. The channel encoder 103 introduces redundancy into this data flow, increasing its rate by adding into it information calculated from the contents of the blocks. The reason for channel coding is to allow the detection or even the correction of signal errors introduced later during transmission. The output of the channel encoder 103 consists of code words of 456 bits each. Exactly one code word is produced from each block of input information for the channel encoder.
The code words that come from the channel encoder 103 are input to the interleaver/burst formatter 104 for mixing up the bits of several code words in a predetermined fashion and organising them into bursts. The aim of interleaving is to decorrelate errors that will potentially occur in the transmission so that the resulting erraneous bits will be distributed into essentially randomised positions in several code words instead of corrupting a sequence of successive bits in a single code word. Most interleaving methods that are currently used are diagonal, meaning that bits from consecutive code words are cross-distributed so that certain bits of the later codeword come earlier in the interleaved data stream than certain other bits of the former codeword. In the GSM arrangement, the bits from a certain full-rate speech channel code word are spread over a period of 8 bursts so that 57 bits from the code word go into each burst. Also other interleaving schemes are used in GSM depending on the nature of the information to be interleaved (speech, data, access request etc.).
Remaining within our full-rate speech example, the burst formation part of the interleaver/burst formatter 104 inserts 57 bits from a certain B:th code word into the odd-numbered bit positions of a burst and 57 bits from the (B+1):th code word into the even-numbered bit positions of the burst. It adds 2 so-called stealing flag bits to get an entity of 116 interleaved bits. Additionally it adds three zero bits (called the tail bits) at the beginning and end thereof as well as a so-called training sequence of 26 bits exactly in the middle. At the output of the interleaver/burst formatter 104 the flow of information consists therefore of bursts of 148 bits altogether. For the description to be consistent throughout this patent application, the bits of a GSM burst will be called symbols in the following. Additionally the burst will be denominated as a digital burst while it is still in digital form.
The ciphering block 105 performs a logical exclusive-or operation between the coded data symbols of a digital burst and a certain pseudo-random bit sequence in order to impede the unauthorised reception of the transmitted data. The tail symbols, the stealing flag symbols and the training sequence are not ciphered. After ciphering the digital bursts are input into a modulator/upconverter 106 that transforms each digital burst into a sequence of a radio-frequency analogue oscillating signal, which is amplified in an amplifier 107 and conducted into an antenna 108 for transmission. Because of its close connection with the digital burst, the analogue signal sequence is also known as a burst; for clarity it can be further specified as a transmission burst. Several filtering operations take place inside the modulator/upconverter 106 and between it and the antenna 108; for graphical clarity the respective filter blocks are omitted from FIG. 1. In the TDMA scheme of GSM each speech channel may use a single time slot in a cyclically repeated frame of eight consecutive time slots. The transmitter transmits one transmission burst in an allocated time slot of each consecutive frame during the active connection.
A receiver chain for receiving, demodulating and decoding the data transmitted by the transmission chain of FIG. 1 would consist of a receiving antenna for receiving the radio signal, some filters and amplifiers for filtering and amplifying the received signal, a downconverter/demodulator or an equalizer for converting the transmission burst into digital form on baseband frequency, a deciphering block for converting the ciphered bits into plain data, a burst deconstructing/de-interleaving block for exctracting the data bits and removing the interleaving, a channel decoder for removing the channel coding, and a speech decoder/D/A converter for converting the decoded digital signal into an analogue signal from which the original speech may be reproduced by a loudspeaker. The operation of the blocks in the receiver chain is approximately the inverse of that of the respective blocks in the transmitter chain.
Minor changes are required in the above-explained functions of the transmission and reception chain blocks for other transmission modes than full-rate speech. These changes are known to the person skilled in the art from the GSM specifications published by ETSI (European Telecommunications Standards Institute) and e.g. from the book Michel Mouly, Marie-Bernadette Pautet: “The GSM System for Mobile Communications”, published by the authors, ISBN 2-9507190-0-7, Palaiseau 1992.
The transmission chain of FIG. 1 is basically applicable also for EDGE transmissions, although the use of higher data rates would necessitate changes in the function of the blocks. Data requiring a higher data rate would most probably originate from a different source than a microphone and a speech encoder, for example a camera and a video encoder. The channel encoder block would operate according to the EDGE channel encoding scheme and, together with the interleaver/burst formatter, ciphering block and the modulator part of the modulator/upconverter, it would have to operate much faster than in basic GSM. The channel encoder block would also be capable of changing the amount of applied channel encoding according to link adaptation commands.
The most radical difference would result from the different modulation method. In the 8 PSK modulation scheme of EDGE, three consecutive bits in the formatted digital burst are mapped onto one transmission symbol. For this reason already a symbol in the digital burst is said to consist of a group of three consecutive bits instead of one bit as in GSM. During the transmission of a burst, the transmitter will produce transmission symbols with the instantaneous rate of 270 ksymbols/s, which is the same as in GSM; the difference in performance results from the fact that an 8 PSK symbol carries the information equivalent to three bits, whereas in GMSK each symbol only corresponds to one bit.
If the propagations conditions on a radio channel are good, it is possible to multiplex two simultaneous connections between the base station and GSM or EDGE terminals so that each connection is only allowed to use the allocated slot in every second frame. This arrangement is known as the allocation of a half-rate traffic channel. The general burst structure to be transmitted in an allocated time slot remains the same, but the generation of data to be transmitted, the channel coding and the interleaving/burst formatting schemes have to be adapted accordingly. The same approach can be extended to quarter-rate traffic channels, where a certain connection is only allowed to use the allocated time slot in every fourth frame, and even to eighth- or octave-rate traffic channels, where a certain connection is only allowed to use the allocated time slot in every eighth frame.
The drawback of the known multiplexing methods is that they tend to decrease the effective interleaving depth. A full-rate traffic channel has an effective interleaving depth of eigth frames, a half-rate traffic channel has an effective interleaving depth of four frames, and so on until the eighth- or octave-rate traffic channel has an effective interleaving depth of only one frame. Reducing the interleaving depth is synonymous to increasing sensitivity to propagation errors, so in order to achieve a certain QoS or Quality of Service the signal to noise (S/N) ratio or carrier to interference (C/I) ratio associated with the connection should increase correspondingly. This not a feasible assumption, because simulation has shown that for example an octave-rate traffic channel would require a C/I ratio of over 30 dB. The severity of the problem is underlined by the fact that every one of the multiplexed connections should simultaneously experience the same excellent propagation conditions, which is not probable since the locations of the users within the cell may vary considerably.
A straightforward solution for providing more robustness against propagation errors on the reduced-rate traffic channels would be to extend the interleaving scheme depending on the channel rate so that the bits of a certain code word would be spread over a larger number of bursts. However, this leads to unacceptably long delays in the interleaving and deinterleaving stages.
From the applicant's U.S. patent applications Ser. Nos. 60/144,307, 60/144,491 and 60/144,723, which are not publicly known at the priority date of the present patent application, there is known a method and an arrangement where the known burst structure is radically changed so that a single time slot actually comes to house two different, temporally consecutive bursts separated from each other by a guard period. This solution relieves the requirements for excessively good carrier to interference ratio, but it has the drawback of necessitating the specification and implementation of a completely new burst structure, which makes it unattractive to the designers of transceiver equipment.